1. Field of the Invention
The present invention relates to a printing apparatus, particularly to a printing apparatus for forming a image using a signal modulated for each picture element, such as a laser printer, and more particularly to a printing apparatus capable of changing a pixel density (a number of pixels in a unit length).
2. Description of the Prior Art
A laser printer is hereinafter described as an example of a printing apparatus. A laser printer forms an image by a large number of pixels arranged in a matrix. In the laser printer, a laser beam modulated to be applied or shut off dependent on information serves to form a latent image on a photoconductor drum and the image is caused to be visible through development of toner, whereby it is transferred onto plain paper and then fixed. Since a laser printer is capable of modulating a laser beam at high speed, printing of letters or graphics can be effected with high speed and high quality (high density). Accordingly, laser printers are widely utilized as output apparatus of various data processing systems or picture forming systems using computers.
Images outputted from a host computer or the like have different densities and in order to print images adequately upon receipt of those outputs, it is necessary to control a pixel density in a printer in a variable manner according to those outputs. In addition, in order to change a size of letters to be printed by using a character generator having an identical arrangement of pixels, the pixel density in the printer needs to be variable.
In order to meet such requirements, laser printers having a variable pixel density have been proposed (for example, Japanese Patent Laying-Open Gazette No. 198076/1984).
However, such a conventional printer is still inconvenient to use.
More specifically, in order to detect abnormality in a laser beam caused by a laser diode or a polygon mirror, the scanned laser beam is detected by a photo sensor for each scanning and determination is made as to whether an output from the photo sensor is provided in a given period, that is, whether the laser beam passes on the photo sensor in the given time. However, in the prior art, the period (time) serving as a reference for determining the state of the laser beam is fixed and accordingly abnormality of the laser beam cannot be correctly detected in a laser printer having a variable scanning period for the laser beam. More specifically, since the period serving as a reference for determination needs to be set longer than the longest period (time) of the scanning cycles of the laser beam, detection in a critical state cannot be made if a scanning cycle is short and, even in an abnormal case in which the scanning period is longer than that in a normal state, it happens that the abnormality cannot be detected.
In addition, if a pixel density is to be changed, a period of an unstable scanning cycle exists during a time from the start to the end of the change and since change of the scanning cycle is generally effected through mechanical operation, a time as long as several seconds to several tens of seconds is required until the change is completed. Consequently, a conventional laser printer has a disadvantage that detention of abnormality occurs in spite of no abnormality in the laser beam due to unstable setting of the scanning cycle at the time of changing the pixel density.
In addition, a laser printer has a process similar to an electrophotographic process and accordingly a density of an image printed finally on plain paper changes dependent on an amount of toner. Therefore, in order to maintain the density of the image in the optimum condition, it is necessary to control the amount of toner. For that purpose, an image of a mark for density detection is formed on a photoreceptor and after it is developed, it is read by a density reader, so that the density thereof is detected. The mark for density detection is formed on the photoreceptor at a position where it does not overlap with an image area of an original image and control is made so that the mark is constantly formed at the fixed position.
If the pixel density is changed from the start of a new page of images successively processed by a host computer or the like, control data for a request for change of the pixel density as well as image data of the successively processed images is outputted from the host computer to the laser printer. However, if the pixel density is changed while a mark for density detection or an image of an image area is being formed, the image cannot be formed normally and therefore the pixel density must not be changed until the formation of the images is completed. Consequently, in the prior art, the host computer needs to always observe the state of the laser printer and a request for change of the pixel density needs to be issued in view of timing when the image is not being formed. Thus, signal processing for change of the pixel density on the side of the host computer is complicated and the processing speed is decreased. To the contrary, if the mark for density detection or the image of the image area is formed while the pixel density is being changed, the image cannot be formed normally, and, accordingly, the image must not be formed until completion of the changing operation of the pixel density. However, in the prior art, operation for forming the image is started during the changing operation of the pixel density and the mark for density detection is not formed normally, causing disturbance in control of the density of the image. To avoid this trouble, the changing operation of the pixel density must not be performed concurrently with the formation of the image and for that purpose, the state of the laser printer should be constantly observed by the host computer and the request for change of the pixel density needs to be issued in view of timing not causing any disturbance in formation of the image. As a result, signal processing for changing the pixel density on the side of the host computer is complicated, causing the processing speed to be decreased.
In the laser printer which has a process similar to an electrophotographic process as described above, in which a latent image of pixels is formed on a photoreceptor drum by means of a scanning beam, a beam detector for detecting the scanning beam is provided at a position in the main scanning direction, ahead of the photoreceptor drum and a laser diode is forced to emit light when the scanning beam passes by the beam detector, whereby a synchronization signal (an SSCAN signal) is obtained. However, if the pixel density (i.e., the printing density) is changed, the scanning speed becomes unstable in a period from the start to the end of the change and it is difficult to apply correct timing to forced emission of light of the laser diode during this period. Thus, the forced emission of light cannot be effected at a fixed position in the scanning direction. In addition, if the pixel density is changed by change of a rotating speed of the polygon mirror, a time as long as several seconds to several tens of seconds is required until the rotating speed of the polygon mirror becomes stable. Accordingly, during such a long time of the unstable state, the laser beam forcedly emitted is applied to the photoreceptor drum to effect unnecessary exposure, resulting in fatigue of the photoreceptor drum or wasteful consumption of toner.